ELABORATION OF FOOD. 575 



Effect of various Rays of the Spectrum. The unlike influence of the 

 different rays of the spectrum is very remarkable. According- to Daubeny 

 and Draper, whose observations have been confirmed by numerous ob- 

 servers, sunlight acts in proportion to its illuminating po'wer in the de- 

 oxidating process, which appears to be just the reverse of what occurs in 

 the reducing action of light upon silver. The yellow rays are almost as 

 powerful as white light ; while the more refrangible rays, blue, violet, 

 &c., have little or no effect on the emission of oxygen, though it is pro- 

 bable they may exert great influence on the chemical transformations 

 which follow that process, and have a directly favourable influence on 

 heliotropic curvatures, periodic movements, currents of protoplasm, &c. 

 (Pfeffer, Baranetsky). In green light the leaves emit carbonic dioxide 

 gas, as in darkness. Diffused light is rich in the more refrangible rays, 

 and hence causes a scanty emission of oxygen. Prillieux, however, asserts 

 that the amount of oxygen emitted by light of different colours is in di- 

 rect proportion to their illuminating-power, and that the effect of the vel- 

 low and red rays in causing the disengagement of oxygen is due to ttteir 

 luminous intensity. A corresponding fact has been noticed with regard 

 to the evaporation of water, so that the two phenomena would appear to 

 be in some way connected. It has been found that starch is formed under 

 white, yellow, or blue light, but in different proportions and with different 

 degrees of rapidity, its formation under the influence of blue light being 

 much slower than under white light. 



Quantity of Oxygen. The quantity of oxygen given off bears a 

 definite proportion to the carbonic dioxide absorbed by a plant ; but 

 excess of carbonic dioxide becomes obnoxious to health. 



Elimination of Nitrogen, &c. It would appear that nitrogen 

 is also given off by plants exposed to sunlight. Draper observed 

 considerable quantities exhaled ; and Cloez and Gratiolet noticed 

 more than was attributable to air accidentally present in the inter- 

 cellular passages. According to Corenwinder the proportion of 

 nitrogenous matter and phosphates gradually decreases from the 

 time of the opening of the leaves till their fall. 



Boussingault asserts that in the case of marsh plants a small proportion 

 of carbonic oxide is exhaled by the green parts of plants, but probably not 

 under normal comditions. 



Elimination of Carbonic Dioxide. When the influence of the 

 sun is withheld from green plants they cease to give off oxygen ; 

 carbonic dioxide is now not absorbed but exhaled, oxygen being 

 absorbed from the surrounding medium. 



Some entire plants destitute of chlorophyll (Fungi and parasites) 

 and certain parts of most others (buds, roots, flowers, germinating 

 seeds, &c.) absorb oxygen at all times and exhale carbonic dioxide, 

 and thus become, like animals, an apparatus for the combustion of 

 carbon and hydrogen. 



The carbonic dioxide given off from the interior of stems, roots, 



